Secret of Twin Asteroid Birth Revealed

Asteroids
are changeable worlds that can split into pieces, creating two
smaller space rocks with separate paths around the sun, a new study
finds. The process can happen non-destructively  just add sunlight
and lots of time.

The
space rock discovery comes from an analysis of 35 so-called "divorced
asteroid pairs" by an international team of astronomers. First
discovered only two years ago, divorced asteroid
pairs
are space rocks that take similar  but separate  paths around
the sun, and have come very close together at some point in the last
million years. Their origins remained a mystery, until now.

"Asteroids
aren't just static boulders floating around in space," said
study co-author Daniel Scheeres of the University of Colorado,
Boulder. "They're constantly evolving over time."

The
research team, led by astronomer led by Petr Pravec of the
Astronomical Institute in the Czech Republic, used several
telescopes around the world to make the asteroid find. They
determined the sizes of these asteroids
by measuring their relative brightness, and studied the spin rate of
each pair with a technique known as photometry.

"It
was clear to us then that just computing orbits of the paired
asteroids was not sufficient to understand their origin," Pravec
said in statement. "We had to study the properties of the
bodies." [Photos
of asteroids in space.]

The
research is detailed in the Aug. 26 issue of the journal Nature.

Twin
asteroid birth predicted

The
asteroids scrutinized in the study were all on the small side,
averaging less than 6 miles (10 km) wide. Researchers found that all
of the asteroid pairs analyzed shared a specific size relationship:
The smaller one was always less than 60 percent as big as its
companion. These measurements fit precisely with a theory Scheeres
developed in 2007, which postulated a way that divorced asteroid
pairs could form.

Scheeres'
theory addressed the nature and destiny of "binary asteroid
pairs"  asteroids that orbit each other as they zoom around
the sun. Unlike "divorced pairs," the two asteroids share
an overall path, orbiting together.

Binary
asteroid pairs are fairly common in the solar system. One way they
can form, astronomers think, is via some long-term solar heating. If
an asteroid is small  less than 6 miles or so in diameter  the
sun
can help break it apart.
Solar radiation blasting one side can cause the space rock to spin
faster and faster over millions of years.

"When
sunlight shines on asteroids, it can spin them up like a propeller,"
Scheeres told SPACE.com.

Most
known asteroids in the solar system are concentrated in a region
between the orbits of Mars and Jupiter called the asteroid belt,
which is about 200 million miles from the sun. But some also extend
into the inner solar system as near-Earth asteroids. Astronomers
estimate there are 1 million asteroids larger than 0.6 miles (1 km)
wide, in the solar system. NASA's infrared WISE space telescope has
discovered more than 25,000 previously
unknown asteroids
in the last six months.

More
observations needed

Many
asteroids are thought to be "rubble piles," rocky bits and
pieces held together by each other's tenuous gravity. If the
solar-induced spin gets fast enough, a chunk on an asteroid's end can
split off.

In
binary asteroid pairs, the theory goes, this chunk sticks with the
bigger, "parent" asteroid, and the two rotate around each
other. But Scheeres' calculations predicted that the "baby"
can break free if it's less than 60 percent as big as the parent. The
result would be two space rocks that take slightly different paths
around the sun  a divorced asteroid pair.

The
new study's findings confirmed that theory and should help
astronomers understand how
asteroids form
and develop  and give them further confidence that many of their
theories and models represent reality.

"It's
one thing to do the math and predict these things," Scheeres
said. "It's another to actually go out and observe them."